Titanium aluminide alloys

ABSTRACT

Alloys based on titanium aluminides, such as γ (TiAl) which may be made through the use of casting or powder metallurgical processes and heat treatments. The alloys contain titanium, 38 to 46 atom % aluminum, and 5 to 10 atom % niobium, and they contain composite lamella structures with B19 phase and β phase there in a volume ratio of the B19 phase to β phase 0.05:1 and 20:1.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Patent application Ser. No.12/331,909 filed Dec. 10, 2008, which claims priority German patentapplication DE 10 2007 060 587.2, filed Dec. 13, 2007, the subjectmatter of these patents is incorporated by reference herein in theirentirety.

FIELD OF THE INVENTION

The invention relates to alloys based on titanium aluminide, inparticular made through the use of casting or powder metallurgicalprocesses, preferably based on γ (TiAl).

BACKGROUND OF THE INVENTION

Titanium aluminide alloys are characterized by a low density, a highrigidity and good corrosion resistance. In the fixed state, they havedomains with hexagonal (α), two-phase structures (α+β) and cubicallybody-centered β phase and/or γ phase.

For industrial practice, alloys based on an intermetallic phase γ (TiAl)with a tetragonal structure and containing minority shares ofintermetallic phase α₂(Ti₃Al) with hexagonal structure in addition tothe majority phase γ (TiAl) are particularly interesting. These γtitanium aluminide alloys are characterized by properties like lowdensity (3.85-4.2 g/cm³), high elastic modulus, high rigidity and creepresistance up to 700° C., which make them attractive as lightweightconstruction materials for high-temperature applications. Examples ofsuch applications include turbine buckets in aircraft engines and instationary gas turbines, and valves for engines and hot gas ventilators.

In the technically important area of alloys with aluminum contentbetween 45 atom percent and 49 atom percent, a series of phaseconversions occur during the solidification from the cast and during thesubsequent cooling. The solidification can either take place completelyvia the β mixed crystal with a cubically body-centered structure (hightemperature phase) or in two peritectic reactions, in which the α mixedcrystal with hexagonal structure and the γ phase participate. Atompercent (at %) of an elemental material in an alloy indicates theproportion of the identified material as 100×[the number of atoms of theidentified elemental material)/(total atoms in the alloy]. This isequivalent to mole percent of the material, or 100×[mole fractionX_(A)], where X_(A)=n_(A)/N_(tot), where n_(a) is the number of moles ofelemental material A in the alloy and N_(tot) is the total number ofmoles of atoms in the alloy.

Furthermore, it is known that aluminum in γ titanium aluminide alloyscauses an increase in the ductility and the oxidation resistance.Moreover, element niobium (Nb) leads to an increase in the rigidity,creep resistance, oxidation resistance, but also the ductility. With theelement boron (B), which is practically insoluble in the γ phase, agrain refinement can be achieved in both the as-cast state and after thereshaping with subsequent heat treatment in the α area. An increasedshare of β phase in the structure as a result of low aluminum contentsand high concentrations of β stabilizing elements can lead to roughdispersion of this phase and can cause deterioration of the mechanicalproperties.

The mechanical properties of titanium aluminide alloys are stronglyanisotropic due to their deformation and breaking behavior but also dueto the structural anisotropy of the preferably set lamellar structure orduplex structure. Casting processes, different powder-metallurgical andreshaping processes and combinations of these production processes areused for a targeted setting of structure and texture in the productionof components made of titanium aliminides.

Moreover, a titanium aluminide alloy, which has a structurally andchemically homogeneous structure, is known from EP 1 015 650 B1. Themajority phases γ (TiAl) and α₂ (Ti₃Al) are hereby distributed in afinely disperse manner. The disclosed titanium aluminide alloy with analuminum content of 45 atom percent (at %) is characterized byextraordinarily good mechanical properties and high temperatureproperties.

Titanium aluminides based on γ (TiAl) are characterized in general byrelatively high rigidities, high elastic modulus, good oxidation andcreep resistance with simultaneously lower density. Based on theseproperties, TiAl alloys should be used as high temperature materials.These types of applications are heavily impaired through the very lowplastic malleability and the low fracture toughness. Rigidity andmalleability, as with many other materials, behave hereby inversely. Thetechnically interesting high-strength alloys are thereby oftenparticularly brittle. Comprehensive examinations for the optimization ofthe structure were performed in order to eliminate these disadvantageousproperties.

The previously developed structure types can be roughly categorized intoa) coaxial gamma structures, b) duplex structures and c) lamellarstructures. The currently achieved development state is represented indetail for example in: Y.-W. Kim, D. M. Dimiduk, in: StructuralIntermetallics 1997, Eds. M. V. Nathal, R. Darolia, C. T. Liu, P. L.Martin, D. B. Miracle, R. Wagner, M. Yamaguchi, TMS, Warrendale Pa.,1996, pg. 531, and M. Yamaguchi, H. Inui, K. Ito, Acta mater. 48 (2000),pg. 307.

The structures made of titanium aluminides were previously mainlyrefined by boron additives, which leads to the formation titaniumborides (see T. T. Cheng in: Gamma Titanium Aluminides 1999, Eds. Y.-W.Kim, D. M. Dimiduk, M. H. Loretto, TMS, Warrendale Pa., 1999, pg. 389;and

Y.-W. Kim, D. M. Dimiduk, in: Structural Intermetallics 2001, Eds. K. J.Hemker, D. M. Dimiduk, H. Clemens, R. Darolia, H. Inui, J. M. Larsen, V.K. Sikka, M. Thomas, J. D. Whittenberger, TMS, Warrendale Pa., 2001, pg.625.)

For further refining and consolidation of the structure, the alloys areusually subjected to several high temperature reshapings throughextruding or forging. Also refer to the following publications:

-   Gamma Titanium Aluminides, Eds. Y.-W. Kim, R. Wagner, M. Yamaguchi,    TMS, Warrendale Pa., 1995;-   Structural Intermetallics 1997, Eds. M. V. Nathal, R. Darolia, C. T.    Liu, P. L. Martin, D. B. Miracle, R. Wagner, M. Yamaguchi, TMS,    Warrendale Pa., 1997;-   Gamma Titanium Aluminides 1999, Eds. Y-W. Kim, D. M. Dimiduk, M. H.    Loretto, TMS, Warrendale Pa., 1999; and-   Structural Intermetallics 2001, Eds. K. J. Hemker, D. M. Dimiduk, H.    Clemens, R. Darolia, H. Inui, J. M. Larsen, V. K. Sikka, M.    Thomas, J. D. Whittenberger, TMS, Warrendale Pa., 2001.

SUMMARY

The present invention resides in one aspect in an alloy which containstitanium, 38 to 46 atom percent (at %) aluminum, and 5 to 10 atompercent niobium, and has composite lamella that contain a B19 phase anda β phase in a volume ratio of B19:β of 0.05:1 to 20:1.

The present invention resides in another aspect in a method for theproduction of an alloy. The method includes providing a composition thatcontains titanium, 38 to 46 at % aluminum, and 5 to 10 at % niobium andsubjecting the composition to a casting or powder metallurgicaltechnique to produce an intermediate product. The intermediate productis subjected to a heat treatment. The heat treatment includes heatingthe intermediate product at a temperature above 900° C. for more thansixty minutes, and cooling the intermediate product at a rate of morethan 0.5° C. per minute.

The present invention resides in another aspect in an alloy made by themethod described herein.

A component may be made from the alloys described herein.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A is an electron photomicrograph of an alloy according to oneembodiment of the present invention.

FIG. 1B is an electron photomicrograph providing a detailed view ofselected lamella structures T of FIG. 1A.

FIG. 1C is an electron photomicrograph of an alloy according to anotherembodiment of the present invention.

FIG. 2A is an electron photomicrograph providing a more detailed view ofa lamella structure T of FIG. 1A.

FIG. 2B is an electron photomicrograph providing a still more detailedview of a lamella structure T of FIG. 1A.

FIG. 2C is a diffractogram derived from FIG. 2B.

FIG. 3 is an electron photomicrograph of a crack in the alloy of FIG.1A.

FIG. 4 is a graph of a plot of force on the vertical axis vs. deflectionon the horizontal axis, for a toughness test of an alloy as describedherein.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment, the present invention provides a titanium aluminidealloy with a fine structure morphology, for example, a morphology in thenanometer range. In another embodiment, the present invention provides acomponent made from a homogeneous alloy.

In one aspect, the present invention provides an alloy based on titaniumaluminides which may optionally be made through the use of casting orpowder metallurgical processes, preferably based on γ (TiAl), using acomposition that contains titanium (Ti), 38 to 42 atom percent (at %)aluminum (Al), and 5 to 10 at % niobium (Nb), and wherein thecomposition comprises composite lamella structures with B19 phase and βphase in each lamella, with a volume ratio of the B19 phase to the βphase in each lamella between 0.05:1 to 20:1. In an optional embodiment,the volume ratio is between 0.1:1 and 10:1.

It has been shown that in some alloys or intermetallic connectionsdescribed herein, composite lamella structures, including compositelamella structures in the nanometer size, are created. The lamellastructures include modulated lamellas made of the crystallographicallydifferent, and alternatingly formed, B19 phase and β phase. The createdcomposite lamella structures are largely surrounded by γ-TiAI.

These types of composite lamella structures can be established in alloysusing known production technologies, i.e. through casting, reshaping andpowder technologies. The alloys are characterized by an extremely highrigidity and creep resistance with simultaneously high ductility andfracture toughness.

Example alloys as described herein can be provided with any of thefollowing titanium-based compositions (wherein titanium makes up thebalance of the at % of each composition):

-   Titanium, 38.5 to 42.5 at % Al, 5 to 10 at % Nb, and 0.5 to 5 at %    Cr;-   Titanium, 39 to 43 at % Al, 5 to 10 at % Nb, and 0.5 to 5 at % Zr;-   Titanium, 41 to 44.5 at % Al, 5 to 10 at % Nb, and 0.5 to 5 at % Mo,-   Titanium, 41 to 44.5 at % Al, 5 to 10 at % Nb, and 0.5 to 5 at % Fe,-   Titanium, 41 to 45 at % Al, 5 to 10 at % Nb, and 0.1 to 1 at % La;-   Titanium, 41 to 45 at % Al, 5 to 10 at % Nb, and 0.1 to 1 at % Sc;,-   Titanium, 41 to 45 at % Al, 5 to 10 at % Nb; and 0.1 to 1 at % Y;-   Titanium, 42 to 46 at % Al, 5 to 10 at % Nb, and 0.5 to 5 at % Mn;-   Titanium, 41 to 45 at % Al, 5 to 10 at % Nb, and 0.5 to 5 at % Ta;-   Titanium, 41 to 45 at % Al, 5 to 10 at % Nb, and 0.5 to 5 at % V;    and-   Titanium, 41 to 46 at % Al, 5 to 10 at % Nb, and 0.5 to 5 at % W.

Each of the titanium aluminide alloys disclosed above can optionallyinclude boron (B) and/or carbon (C). For example, any of the describedtitanium aluminide alloys may include 0.1 to 1 at % boron and/or 0.1 to1 at % carbon. The already fine structure of the alloy is hereby furtherrefined.

Within the framework of the invention, the remainders of the specifiedalloy compositions are made of titanium and unavoidable impurities.

In accordance with the invention, alloys are thus made available, whichare suitable as a lightweight construction material for high temperatureapplications, such as turbine buckets or engine and turbine components.

According to one aspect, alloys as described herein can be producedusing casting metallurgical or powder metallurgical processes ortechniques, or using these processes in combination with reshapingtechniques.

In some embodiments, the alloys with composite lamella structures have avery fine microstructure and a high rigidity and creep resistance withsimultaneously good ductility and fracture toughness with respect toalloys without the composite lamella structures.

It is known that titanium aluminide alloys with aluminum contents of38-45 at % and other additives, for example, refractory elements,contain relatively large volume shares of the β phase, which can also bepresent in a controlled form as B2 phase. The crystallographic latticesof these two phases are mechanically instable with respect to homogenousshearing processes, which can lead to lattice conversions. This propertyis mainly attributed to the anistropic bond ratio and the symmetry ofthe cubically body-centered lattice. The tendency of the β or B2 phasetowards lattice transformation is thus very distinct. Differentorthorhombic phases can be formed through a shear transformation of thecubically body-centered lattice of the β or B2 phase, to which phasesB19 and B33 belong in particular.

Without wishing to be bound by any one particular theory, the inventionis based on the idea of using lattice transformations through shearconversion for an additional refining of the microstructure of thetitanium aluminide alloys. This type of process is not previously knownfor titanium aluminide alloys in scientific literature. In the case ofthe alloys as described herein, the formation of brittle phases like ω,ω′ and ω″, which are extremely disadvantageous for the mechanicalmaterial properties, are also avoided, due to shear conversions.

In some embodiments, the structural refining of the alloys describedherein is achieved without the addition of grain-refining orstructure-refining elements or additives such as boron (B) and thealloys thus contain no borides. Since the borides occurring in TiAlalloys are brittle, they lead to the brittleness of TiAl alloys as of acertain content and generally represent potential crack nuclei inboron-containing alloys.

In another aspect, some alloys as described herein comprise compositelamella structures with the B19 phase and β phase in each lamella,wherein the lamellas are surrounded by the TiAl-γ phase.

In various embodiments, the volume ratio of the B19 phase and β phaseeach in a lamella is between 0.05:1 and 20:1, for example, 0.1:1 and10:1.In some embodiments, the volume ratio of the B19 phase and β phasein a lamella is between 0.2:1 and 5:1, and the volume ratio may bebetween 0.25:1 and 4:1. In certain embodiments, the volume ratio of theB19 phase and β phase in a lamella is between 1:3 and 3:1. For example,the volume ratio may be between 0.5:1 and 2:1. Embodiments having aparticularly fine structure in the alloy composition have a ratio, inparticular the volume ratio, of the B19 phase and β phase in a lamella,between 0.75:1 and 1.25:1, for example, particular between 0.8:1 and1.2:1. In one instance, the volume ratio may be between 0.9:1 and 1.1:1.

Moreover, it is possible in a further embodiment of the alloys accordingto the invention that lamellas of the composite lamella structures aresurrounded by lamellas of type γ (TiAl), preferably on both sides of thelamella.

The alloys are further characterized in that the lamellas of thecomposite lamella structures have a volume share of more than 10%,optionally more than 20%, of the total alloy.

Moreover, the fine lamella-like structure in the composite structuresare retained if the lamellas of the composite lamella structures TiAlhave the phase α₂-Ti₃Al with a volume share of up to 20%, wherein inparticular the (volume) ratio of the B19 phase and β phase in thelamellas remains unchanged and constant.

The alloys according to the invention are suitable as high temperaturelightweight construction material for components that are exposed totemperatures of up to 800° C.

An alloy as described herein can be produced using casting or powdermetallurgical techniques. The casting or powder metallurgical techniquesare used to produce an intermediate alloy product containing thetitanium, aluminum, niobium and optional other components, if any, inthe appropriate proportions. The intermediate alloy product is thensubjected to heat treatment including heating at temperatures above 900°C., preferably above 1000° C., in particular at temperatures between1000° C. and 1200° C., for a predetermined period of time of more than60 minutes, preferably more than 90 minutes, yielding a heat-treatedintermediate alloy product. The heat-treated intermediate alloy productis then cooled with a predetermined cooling rate of more than 0.5° C.per minute.

In one embodiment, the heat-treated intermediate alloy product is cooledwith a predetermined cooling rate between 1° C. per minute and 20° C.per minute, preferably up to 10° C. per minute.

Light (high temperature) materials or components for use in thermalengines like combustion engines, gas turbines, and aircraft engines maybe made of an alloy as described herein, e.g., from an alloy based on anintermetallic bond of type γ-TiAl made through casting or powdermetallurgical processes or techniques and heat treatment.

Accordingly, an alloy as described herein can be used for the productionof a component. To avoid repetitions, reference is made expressly to theabove expositions.

As indicated above, the alloys described herein may be created throughthe use of conventional metallurgical casting methods or through knownpowder metallurgical techniques, and can for example be processedthrough hot forging, hot pressing or hot extrusion and hot rolling.

Examples of composite lamella structures of the type described hereinare shown in the figures. The example composite lamella structures arebased on an alloy comprised of titanium (Ti), 42 atom % aluminum (Al)and 8.5 atom % niobium (Nb).

FIG. 1A shows a picture of a structure alloy, which was taken with thehelp of a transmission electron microscope. The overview picture in FIG.1 shows that the composite lamella structures, which are labeled with Tin FIG. 1, have a striped contrast to the structure of the γ phasesurrounding the structures.

FIG. 1B shows a picture of the alloy structure with a highermagnification, whereby it can be seen that the modulated compositelamella structures (reference letter T) are surrounded by the γ phaserespectively are embedded in the γ phase.

The structures shown in FIG. 1A and 1 b were obtained or set throughextrusion.

FIG. 1C shows a cast structure of the same alloy, i.e., an alloycontaining titanium, 42 at % aluminum, and 8.5 at % niobium, in which acomposite lamella structure (indicated in the Figure by the referenceletter T) is also formed, which is surrounding by the γ phase.

FIG. 2A shows a high resolution illustration of the atomic structure ofthe composite lamella structures above the γ phase. The compositelamella structures are made up of the controlled B19 phase and theuncontrolled β phase, which border the γ phase (in the lower area). Itcan be seen from the picture in FIG. 2A that the composite lamellastructures contain the two crystallographically different phases B19 andβ/B2, which are arranged at separation distances of a few nanometers.The composite lamella structures contain the phases B19 and β, which areboth considered ductile. The volume ratio of the B19 phases to the βphases in a composite lamella structure is 0.8:1 to 1.2:1. Due to theductile phases B19 and β, the structure is mainly made of easilymalleable lamellas, which are embedded in the previously relativelybrittle γ phase.

FIG. 2B shows an illustration of a B19 structure with a magnifiedrepresentation. The corresponding diffractogram, which was calculatedfrom the section shown in FIG. 2B and is characteristic for the B19structure, is shown in FIG. 2C.

FIG. 3 shows an electron-photomicrograph of a crack C in theaforementioned alloy. It can be seen from the figure that the crack C isdiffracted at the modulated composite lamella structures (T) and thatthe composite lamella structures form ligaments that can bridge the edgeof the crack. This type of behavior is considerably different from thecrack propagation in the previously known Ti—Al alloys, in which acleavage fracture occurs in the microscopic dimension observed here. Inthe alloy according to the invention, crack propagation is prevented dueto the formed composite lamella structures.

The fracture toughness of structure important for the technicalapplication was determined with the help of notched Chevron samples inthe bending test at different temperatures. The recorded register curveof such a test is shown in FIG. 4. The indentations marked by the arrowscan be seen in the curve, which indicate that crack propagationintermittently occurs during the loading of the sample, but is stoppedagain and again. Such a behavior is typical for alloys that are made upof a brittle phase (γ phase), in which the relative ductile phases B19and β are embedded.

As mentioned above, the alloys according to the invention can be madethrough the technologies known for TiAl alloys, i.e. via castingmetallurgy, reshaping technologies and powder metallurgy. For example,alloys are melted in an electric arc furnace and are re-melted multipletimes and are then subjected to a heat treatment. Moreover, theproduction methods of vacuum arc casting, induction casting or plasmacasting, which are known for primary cast blocks made of TiAl alloys,can be used for production. After the solidification of casting primarycast material, hot-isostatic presses can also be used as the compressionmethod at temperatures of 900° C. to 1,300° C. or heat treatments in thetemperature range of 700° C. to 1,400° C. or a combination of thesetreatments, in order to close pores and to establish the microstructurein the material as described herein.

Although the invention has been described with reference to particularembodiments thereof, it will be understood by one of ordinary skill inthe art, upon a reading and understanding of the foregoing disclosure,that numerous variations and alterations to the disclosed embodimentswill fall within the scope of this invention and of the appended claims.

1. A method for the production of an alloy, comprising: providing acomposition that comprises titanium, 38 to 46 at % aluminum, and 5 to 10at % niobium; subjecting the composition to a casting or powdermetallurgical technique to produce an intermediate product; andsubjecting the intermediate product to a heat treatment, the heattreatment comprising heating the intermediate product at a temperatureabove 900° C. for more than sixty minutes, and cooling the intermediateproduct at a rate of more than 0.5° C. per minute.
 2. The method ofclaim 1 wherein the heat treatment comprises heating the intermediateproduct at a temperature above 1000° C.
 3. The method of claim 1 whereinthe heat treatment comprises heating the intermediate product at atemperature between 1000° C. and 1200° C.
 4. The method of claim 1wherein the heat treatment comprises heating the intermediate product atsaid temperature above 900° C. for more than 90 minutes.
 5. The methodof claim 1 wherein the heat treatment comprises heating the intermediateproduct at a temperature above 1000° C. for more than 90 minutes.
 6. Themethod of claim 1, comprising cooling the intermediate product at a rateof 1° C. per minute to 20° C. per minute.
 7. The method of claim 1,comprising cooling the intermediate product at a rate of 1° C. perminute to 10° C. per minute.